We report a detailed synchrotron x-ray-scattering study of the structural fluctuations near the spin-Peierls transition in CuGe${\mathrm{O}}_3$. Below the transition temperature $T_{\mathrm{SP}}$, the superlattice peak profile is resolution limited and the peak intensity, which is proportional to the order parameter squared, is well described by a simple power law, ${(1-\frac{T}{T_{\mathrm{SP}}})}^{2\beta }$. The fitted value of $\beta$, 0.33(3), agrees quantitatively with previously reported results from neutron-diffraction measurements. Above the transition temperature $T_{\mathrm{SP}}$, pretransitional lattice fluctuations are observed within about 1 K above $T_{\mathrm{SP}}$. The length scale of these fluctuations is about an order of magnitude larger than that characterizing the bulk critical fluctuations. The line shape of these large length-scale fluctuations is consistent with a Lorentzian-squared form. The measured critical exponents associated with the large length-scale fluctuations are $\nu =0.56\mathrm{}\left(9\right)\mathrm{}$, and $\overline{\gamma }=2.0\mathrm{}\left(3\right)\mathrm{}$. Similar large length-scale fluctuations have been observed at the structural transitions in some perovskites and the magnetic transitions in holmium and terbium. We suggest that in CuGe${\mathrm{O}}_3$ the large length-scale fluctuations reflect the disconnected susceptibility originating from random field Ising-type local defects.

• Received 23 August 1995

DOI:https://doi.org/10.1103/PhysRevB.52.15420